Numerous attempts have been made to extend the low frequency performance of small loudspeaker systems. However, none of these have so far proved to be successful commercially.
In GB2146871B there is described a technique for eliminating or substantially eliminating pressure variations in an essentially closed volume, such as the interior of a loudspeaker cabinet. The essentially closed volume of the cabinet is effectively reduced in volume when the loudspeaker cones make excursions into the interior of the cabinet. Such excursions tend to increase the pressure in the cabinet and these pressure variations are reduced by locating a mass of material within the cabinet, the material being adsorbent to the gas or vapour in the cabinet. The adsorbing material can be a mass of activated charcoal or carbon in granular form. The granules are described as being held in place by a mesh-like support structure which can be a moulding of plastics material or formed from expanded metal sheet. This can be lined with a porous fabric such as filter paper to prevent small granules from passing through the support structure.
In GB2146871B it is mentioned that it is desirable to keep the carbon granules free from moisture. Methods of attempting to achieve this described in the aforesaid patent specification include the provision of a moisture impermeable diaphragm located within the box between the granules and the speakers, and also the use of a moisture barrier to prevent moisture from entering the region of the mass of carbon granules.
U.S. Pat. No. 5,857,340 discloses the use of carbon for preferential adsorption of one of a mixture of gases in the acoustic resonator of a thermoacoustic resonator.
U.S. Pat. No. 5,080,743 discloses a method of forming an acoustic diaphragm formed wholly from carbonaceous materials, including crystals of graphite.
Although the use of adsorbent materials in acoustic enclosures may be beneficial, there exist problems with the practical use of such materials.
When a loudspeaker cone moves backwards, the air in the box is compressed slightly. In a conventional loudspeaker this results in a pressure increase which acts to impede the movement of the cone. In a cabinet in accordance with the invention, the pressure increase is smaller because some of the air molecules are momentarily joined to the surface of the carbon granules (adsorbed). So the impedance to motion is significantly reduced. When the cone moves forwards the air molecules are desorbed by the resulting pressure decrease.
One can think of this adsorption as a (temporary) reduction of air density. The acoustic compliance of air in the loudspeaker cabinet is given by:CA=VB/ρc2where VB is the nett enclosure volume
ρ is the density of air
c is the velocity of sound in air
Therefore a reduction in density produces an increase in compliance, equivalent to enlarging the enclosure.
This stiffness reduction or compliance enhancement can be as much as four times or more under optimum conditions. Factors of 1.5 to 3 are readily achievable in practice.
The compliance enhancement achieved with the present invention is principally effective at low frequencies, as shown in FIG. 1. At higher frequencies than about 200 Hz performance deteriorates because the cycle time becomes too short for adsorption and desorption fully to take place.
There is a strong relationship between the tendency of an activated carbon to adsorb air and its tendency to adsorb water vapour. Adsorption of water vapour adversely affects compliance enhancement because the water molecules block the pores and prevent air adsorption. Therefore the carbon must be kept as dry as possible, and its “water uptake” should be minimal.